The present invention relates to a method of making a golf ball. More particularly, it relates to a method of making a golf ball comprising a step of covering polyurethane cover on the surface of a core.
Recently, ionomer resin has been widely used as a cover of golf ball, because the ionomer resin has superior in rebound characteristics, durability, processability and the like. However, since the ionomer resin has high stiffness and hardness, It is inferior in shot feel and controllability when compared with balata (trans-polyisoprene), which is a conventional cover material for thread wound golf balls.
Polyurethane has been also conventionally used as another cover material for golf balls. The golf balls using the polyurethane cover have high resilience when compared with golf balls using the ionomer cover, and have shot feel and controllability as good as golf balls using the balata cover. However, since there has been problem that it is difficult to control a reaction of the polyurethane unlike the ionomer resin, there has been various suggestions of a method of making a golf ball using the polyurethane cover, particularly a method of covering the polyurethane cover (Japanese Patent Kokai Publication Nos. 74726/1976, 261478/1990, 47553/1996 and the like).
In Japanese Patent Kokai Publication No. 74726/1976, a method comprising the steps of forming two half shell blanks of polyurethane, covering the half shell blank on a core to form polyurethane cover by half, and covering the other on the core to complete polyurethane cover is described. In Japanese Patent Kokai Publication No. 261478/1990, a method comprising the steps of forming polyurethane cover having a smooth surface, and then forming dimples on the cover by compression molding is described. However, there was problem that the reaction of the polyurethane was not uniform in the both methods, because the reaction is not sufficiently controlled.
In Japanese Patent Kokai Publication No. 47553/1996, a method comprising the steps of placing polyurethane in a selected state of gel in a first mold; covering a half of the surface of the core, of which the surface is coated with latex, with the polyurethane cover in the first mold positioned below the core; placing polyurethane in a selected state of gel in a second mold as described above; and covering the other half of the surface with the polyurethane cover is described. In the method, the surface of the core is coated with latex by dipping the core in latex in order to avoid of permitting air to pass through it into the interstices in the windings of the wound core or allowing the rubber strands to unravel. There is problem to degrade rebound characteristics and spin amount, because the latex layer is a cushion layer between the core and cover and it is difficult to penetrate the cover into the core by the presence of the latex layer.
A main object of the present invention is to provide a method of making a golf ball having excellent initial velocity and excellent spin performance by accomplishing uniformly thickness of the cover, and the effectively transmission of applied force thereon at the time of hitting because of preventing the thread rubber from unraveling, and penetrating the cover material into the interstices of the windings of a thread rubber in case of using the thread wound core.
According to the present invention, the object described above has been accomplished by using a method of making a golf ball comprising the steps of:
putting polyurethane composition for the cover into a preheated first half mold, followed by allowing said polyurethane composition to be made to a given gel state,
placing the core on a core half mold having a semi-spherical cavity with a diameter enough for holding the core therein, and turning the first half mold upside down so as to position it above the core half mold,
clamping the first half mold and the core half mold to press mold with heating,
forming an intermediate molded article of golf ball, which a half of the surface of the core is covered with the polyurethane cover,
putting polyurethane composition for the cover into a preheated second half mold, and allowing said polyurethane composition to be made to a given gel state in the same manner as the first half mold, followed by positioning the second half mold below the first half mold with keeping the intermediate molded article in the first half mold,
clamping the first half mold and the second half mold to press mold with heating, and
covering the other half of the surface of the core with the polyurethane cover,
thereby providing a method of making a golf ball having excellent initial velocity and excellent spin performance by accomplishing uniformly thickness of the cover, and the effectively transmission of applied force thereon at the time of hitting because of preventing the thread rubber from unraveling and penetrating the cover material into the interstices of the windings of a thread wound core in case of using the thread wound core.
This object as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawings.